Planar light-wave circuits, also known as planar light-wave chips (PLCs), are optical devices having optical components and networks disposed monolithically within stack(s) of optical thin films and supported by a common mechanical substrate such as a semiconductor or glass wafer. PLCs are typically designed to provide specific transport or routing functions within fiber-optic communications networks. These networks are distributed over a multitude of geographically-dispersed terminals and commonly include transport between terminals via single-mode optical fibers.
Wavelength-division multiplexing (WDM) is a commonly employed technology within telecommunication systems that provides transmission of multiple optical signals on a signal optical fiber by using different wavelengths to carry different signals. In the WDM system, each optical carrier signal is transmitted within a narrow wavelength band centered around a center wavelength. Each band is commonly referred to as an optical channel and is generally characterized or referred to by a single center wavelength (λx).
The WDM system uses a multiplexer to join the optical carrier signals together for transmission over the single optical fiber while a demultiplexer is used to split the optical carrier signals apart. The multiplexer takes the optical signal having different channels and combines them for transmission over the single optical fiber. The demultiplexer performs the reverse application and splits the signal into multiple optical signals. In this regard, WDM systems allow capacity expansion of the network without having to lay out more optical fibers since capacity of a given link can be expanded by simply upgrading the multiplexers and demultiplexers.
Optical filters serve as components in the WDM systems that provide the signal processing functions needed in multiplexing/demultiplexing, balancing of signal power, adding and/or dropping of channels, and the like.
The design goal of an optical filter for a WDM system application is to provide a passband having a wide, nearly flat top with minimum insertion loss and rapid rolloff on the band edges, while minimizing chromatic dispersion across the passband.
Ring-assisted MZ filters are currently being practiced in the art to provide wide, nearly flat top passbands with high extinction ratios. See, Jinguji, K. et al. “Optical Half-Band Filter”, J. Lightwave Technol. Vol. 18, 252-259 (2000); See, Wang, Qi, et al. “Design of 100/300 GHz optical interleaver with IIR architectures”, Optics Express, Vol. 13, (March 2005). However, there is difficulty in minimizing the chromatic dispersion across the passband of a ring-assisted MZ filter.
Within communication systems, chromatic dispersion in an optical fiber or within other optical components limits pulse propagation distances and/or bit rates. Dispersion is generally wavelength-dependent and causes a broadening of the optical signal as it propagates along the length of the optical fiber. Different wavelengths travel at different speeds along the length of the optical fiber, and as signals broaden due to dispersion, it may become difficult to distinguishing one pulse from another.
In light of the foregoing, there is a need to reduce the amount of chromatic dispersion in the optical signal to increase pulse propagation distances and/or bit rates.